Anything taken to the extreme is not good. Too much negative can bring a person down completely and too much positive can bloat ones head, to the point of being delusional. A bit like Idols I suppose... where 90% of the people that audition are just hopeful wannabe's who have been falsely told their entire lives that they sing like angels, when realistically they cant really tell A from F.
I belive in constructive crit. Where, if for example, my teacher tells me I play something badly, I expect him to tell me how I can improve. This way, even if I am a little down after my lesson, I still see hope for my playing from his advice... and it always feels good to come back the next week after working on the section in question and being told I play it well. (he's not someone who will give unnecessary compliments, so when he does, you know you really do play well!)
Maybe you should find a teacher. Because sometimes when you practise, you dont hear things that need to be done, that others, listening, would. (even the best of them need it sometimes!)
An alternative is, record yourself, and be your own worst critic. It helps too.
Anyway, dont be down. You can probably do things that your friends could only dream of!
Its about the music... after all. Remember that. Its all that matters.